Epilepsy is a disorder of central nervous system hyperexcitability and synchrony that affects an estimated 50 million people worldwide. Approximately 30% of epileptic patients do not achieve sufficient control of their seizures with currently available treatments. Moreover, the percentage of patients that have treatment-resistant seizures has not significantly changed in the last 40 years indicating a continued need to identify novel treatment options for patients with refractory epilepsy. Mutations in the voltage gated sodium channels (VGSCs) SCN1A, SCN2A, and SCN3A are associated with several epilepsy subtypes, including genetic (generalized) epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (DS). In striking contrast, we have observed increased seizure resistance in mice with mutations in the VGSC Scn8a. Furthermore, the co- segregation of an Scn8a mutation was sufficient to restore normal seizure thresholds and lifespans to Scn1a knockout mice that serve as a model of DS, which is a catastrophic, early-life refractory epilepsy syndrome. These findings raise the possibility that selective targeting of SCN8A might be therapeutic in patients with refractory epilepsy. However, one limitation with our previous experiments was that mice expressed the Scn1a and Scn8a mutations from birth, and consequently the protective effect may, in part, have been due to compensatory changes during development. Since patients with epilepsy only seek treatment after seizure onset, it is important to establish whether reducing Scn8a expression levels after seizure onset will also ameliorate seizure severity. As a result, this proposal will investigate the effect of reducing Scn8a expression after seizure onset and during epileptogenesis. Mice with a floxed Scn8a allele will be crossed to a tamoxifen-inducible Cre line in order to irreversibly delete Scn8a following tamoxifen administration in adult mice. Aim 1 will use immunocytochemistry to evaluate the cell types that express Scn8a and Scn1a in the mouse forebrain and will examine the effect of reducing Scn8a expression in adult, epileptic, Scn1a knockout mice. Aim 2 will determine the effect of reducing Scn8a expression in a mouse model of temporal lobe epilepsy (TLE), the most common form of refractory epilepsy. Seizures that originate in the temporal lobe will be induced by intra-hippocampal injection of kainic acid. Aim 2 will provide important information about the role of Scn8a in preventing epileptogenesis (the process by which spontaneous seizures arise) and the effect of reducing Scn8a after seizure onset. Seizure activity will be measured by electroencephalographic analysis and histology will be performed to assess changes in hippocampal morphology. This study will advance our understanding of the role of VGSCs in epilepsy and further examine the feasibility of targeting Scn8a as a treatment for refractory epilepsy. These results will have important clinical implications and will facilitate additional translational studies. PUBLIC HEALTH RELEVANCE: Epilepsy affects an estimated 50 million people worldwide and approximately 30% of epileptic patients do not respond to currently available treatments, indicating a continued need for the development of new therapeutics. This proposal will investigate the therapeutic potential of targeting the voltage-gated sodium channel gene Scn8a in treatment-resistant forms of epilepsy. )